The current optical circuit switching (OCS) techniques are based on a reservation of the resources prior to the sending of the data. Resource reservation methods have been specified by standardization bodies such as the IETF in the specification of the GMPLS (Generalized Multi-Protocol Label Switching) control protocol or the ITU within the ASON (Automatically Switched Optical Network) control plane.
The reservation can, for example, be made as follows: the reservation of resources in each node of the connection is initiated at a source node on reception of a connection set-up request. The reservation relies on a signaling mechanism implemented using a signaling protocol. This mechanism consists in sending a reservation request message (“set-up”) from the source node to the destination node and then returning a reception acknowledgement message from the destination node to the source node along the same path as in the outbound direction. On reception of the reception acknowledgement at the source node, the connection is set up and the data are transferred to the destination node over this connection. The resources available for the current connection are identified in each node as the reservation request message passes through each node. This information can be collected by the reservation request message.
The choice of the connection route (succession of the nodes ABCD) can be made by the source node. In this case, the source node must implement a routing algorithm on the basis of the topology that it will have worked out from other control messages (routing protocol) for example. The explicit route must then be indicated in the reservation request and reception acknowledgement messages in order for the nodes receiving these messages to know where said messages have to be transferred to. The choice of the connection route (succession of the nodes ABCD) can also be made hop by hop, each node computing the next jump on the basis of its knowledge of the topology. In this case, the request messages must contain the destination node and update the route so that the reception acknowledgement message correctly uses the same nodes as in the outbound direction (a combination of the two solutions is also possible).
In order to correctly separate the control processes of the network from the connection creation processes on the optical layer, the control plane and transfer plane formalism is conventionally used. Such a formalism is schematically represented in FIG. 2. A telecommunication network 1 comprises node elements A, B, C and D interlinked by optical links. Each node is present both in the transfer plane TP, because of the optical resources that it controls, and in the control plane CP, because it has functions that make it possible to understand and process the messages from the control plane.
The signaling messages are exchanged over the control plane CP whereas the resources are reserved in the transfer plane TP. As schematically represented in FIG. 2, the physical media LcAB, LCBC, LCCD of the control plane CP are not necessarily identical to those LTAB, LTBC, LTCD of the transfer plane TP. In other words, a control link between two nodes does not necessarily pass through the same optical fiber medium as the link of the transfer plane. Furthermore, the nodes do not necessarily have knowledge of the respective physical media of the links of the control plane and of the transfer plane.
The reservation of the resources by exchanges of messages from node to node make it possible to distribute the control of the optical layer in each node (distributed case), unlike the case where the reservation request and reception acknowledgement messages are sent and received by a single centralized entity (centralized case). The reservation of the resources can be performed in the nodes on the basis of the availability of the resources on reception of the reservation request messages (outbound reservation) or of the reception acknowledgement messages (return reservation).
Concerning FIGS. 1A and 1B, a route ABCD of an optical network 1 determined by the control plane of a telecommunication network 1 is considered. In order to transfer data from a node element A to a node element D, the node element A sends a signaling message comprising a reservation request REQ to the node element D via the path ABCD. FIG. 1A illustrates the case of a reservation in the outbound direction, according to which the nodes of the path ABCD initiate the reservation on reception of the reservation request message REQ. The destination node element D sends a signaling message RES to the source element A indicating that the reservation is effective. This message RES is received and retransmitted by all the intermediate nodes. The resources will then be released only on reception of a signaling message requesting the release of the resources. A period PU is then considered which corresponds to the useful period during which the resources are used for the transfer of the data and a reservation period PR corresponding to the total period during which the resources are reserved by the nodes of the path ABCD in order to transfer these data. It has been found that the data reservation time PRA is much greater than the useful reservation time and that the unnecessary reservation time is not negligible, in particular in the case where the useful period PU is of an order of magnitude similar to the signaling time. It will be noted that this unnecessary reservation time is all the greater when the data transfer time approximates to a signaling time, that is to say, a time encompassing the request processing times by the node elements on the route ABCD and the request propagation times over the optical links between the node elements A, B, C and D.
FIG. 1B illustrates the case of a return reservation, which means that the nodes that make up the route ABCD make the resource reservation effective only on reception of a reception acknowledgement message RES sent by the destination node element D in response to the request REQ. The result of this is that the reservation period PRB is less than the time PRA and therefore that the unnecessary reservation time is shorter.
In both cases, the reservation of the resources is effective before the start of the transfer of the data over the route ABCD, which leads to an occupancy of the resources during a period in which no traffic uses them. Similarly, the resources are released only after the reception of a release message, and the nodes managing their resources have not a priori indication concerning the moment when this message will arrive. In the case where the data propagation time approximates to the signaling time, these “unnecessary” reservation times are not negligible and limit the speed of sequencing of the reservations.
As a result, the current resource reservation techniques are poorly suited to a dynamic traffic consisting of a succession of short and frequent reservation periods. In such a context, it could prove useful to set up the connections on the basis of the requirements, whether in the network core or at a terminal element.
To improve the situation, the principle of reserving the resources in a delayed manner for a determined time (“Just Enough Time”) has been described in the prior art, in particular in the document entitled “Improved Dynamic Lightpath Provisioning For Large Wavelength Division Multiplexed Backbones”, by H. Kong and C. Phillips, in the Journal of Lightwave Technologies review, vol. 25, No. 7, pp. 1693-1701, in 2007. However, such a document does not explicitly describe how to compute the useful delay to be taken into account at each node element.